1. Field of the Invention
The invention relates to induction machine rotors and more particularly, rotor slots for retaining rotor bars and methods for forming rotor slots for high speed motors that utilize non-laminated, homogeneous rotor bodies.
2. Description of the Prior Art
Induction machines, such as motors, are constructed with conductive rotor bars that are arrayed within rotor slots. The slots are axially aligned about the rotor body outer circumference. Many motors have laminated rotor bodies that, are formed from a stack of individual stamped metal laminations. In laminated rotor bodies it is often cost effective to stamp cross sectional profiles of rotor slots in each lamination and thereafter forming the individual elongated rotor slots by aligning the stampings axially.
FIG. 1 depicts schematically a known induction motor 20, having a housing 22, stator 24 and bearings 26 it that capture rotating rotor 30. The rotor 30 includes a shaft 32 onto which is concentrically coupled rotor body 34. The rotor body 34 outer circumference retains a plurality of axially oriented rotor bars within integrally formed rotor slots 40. As is shown in FIGS. 2 and 3, typical respective rotor bar 36, 36′ cross-sectional configurations have narrow necks 37, 37′ on their outer peripheries that mate with corresponding rotor slot tabs 42, 42′ formed within the rotor slot 40, 40′. The tabs 42, 42′ radially constrain the rotor bars 36, 36′. The radially constrained rotor bars 36, 36′ will remain in the rotor slots 40, 42′ when centrifugal forces are imparted on them by acceleration of the spinning rotor 30, provided that the tab structures 42, 42′ have sufficient strength and elasticity to resist the generated centrifugal forces. The centrifugal forces increase exponentially with rotor rotational speed. Often in the past rotor bars have also been additionally constrained radially by bonding them in their corresponding rotor slots with adhesives, such as epoxy, friction-interference fit, soldering, brazing, shimming and diffusion brazing.
Stamped rotor laminations that incorporate rotor slots are generally constructed of electrical steel with a generally low tensile strength. In higher speed motor applications rotor slot tabs 42, 42′ formed in rotor laminations do not have sufficient strength to restrain rotor bars, even when combined with additional adhesive or brazing constraints.
One past solution to restrain rotor bars in higher speed motors is shown in FIGS. 4 and 5. The rotor body 34″ is constructed of homogeneous steel, having a higher tensile strength than electrical steel laminations that is capable of resisting higher centrifugal forces imparted on the rotor bars 36″ when the rotor is spinning at high speeds. Rotor slots 40″ are plunge-machined in the outer circumference of the rotor body 34″ with a milling machine ball cutter C, that creates a U-shaped cross-sectional profile with parallel sidewalls oriented parallel to the rotor body radial axis. Lacking any kind, of mechanical interference fit between the rotor bars 36″ and rotor slot 40″ that is analogous to the rotor slot tabs 42, 42′ of FIGS. 1-3, the rotor bars 36″ are bonded to the rotor slot 40″ by a relatively expensive diffusion brazing process that results in a strong brazing layer 44″ between the rotor bar and rotor slot.
It is desirable to restrain rotor bars in rotor slots by resisting centrifugal forces imparted on them with a mechanical, restraint analogous to the rotor slot tabs 42, 42″ used in laminated rotor bodies. However, it is difficult to construct such tabs in relatively hard, homogenous ferromagnetic metal rotor bodies.
Thus, a need exists in the art for a rotor body having machined in place rotor slots that are capable of radially restraining rotor bars through mechanical interference, with or without additional constraining methods, such as adhesive bonding or brazing.